1. Field of the Invention
The present invention relates to a motor current detection circuit, and particularly to a motor current detection circuit for monitoring current flowing through a motor so as to detect an overloaded state of the motor.
2. Description of the Related Art
In order to monitor current flowing through a motor so as to detect an overloaded state of the motor, a shunt resistor is serially connected between the motor and ground such that an electric potential difference is produced between both ends of the resistor. The electric potential at a point between the motor and the resistor is compared with a reference electric potential Vref by using a comparator so as to detect an overloaded state of the motor based on output from the comparator.
Due to the characteristics of a motor, the motor current fluctuates so as to repeatedly increase and decrease within a certain range depending on the number of the poles and the rotational speed (the range becomes more narrow as the number of poles increases). Accordingly, in a case in which the motor current increases on the whole over time along with the above-mentioned fluctuations as shown in FIG. 5, and the reference electric potential Vref is set to a value corresponding, for example, to an allowable load, the output Oc of the comparator repeatedly fluctuates between a high level state (hereinafter referred to as "Hi") and a low level state (hereinafter referred to as "Lo"), as shown in FIG. 5. Due to this fluctuation, chattering which corresponds to the fluctuation in the current occurs in the output Oc, i.e., the detection output. Since the motor current repeatedly increases and decreases within a certain range as mentioned above, a drawback arises in that setting of the reference electric potential Vref for preventing the occurrence of chattering is difficult. Although addition of a so-called chattering prevention circuit in which the motor current is detected after fluctuation of the motor current abates is considered as a possible solution, the addition of the chattering prevention circuit causes the drawback of producing delays in the detecting operation.